This error is due to a communication error in the printer. Since there can be several causes for this, we recommend to first reboot the Ultimaker S5 by using the on/off button.
Please contact your local service provider for help and send them the log files of your Ultimaker S5. If a USB stick was inserted into the Ultimaker S5 when the error appeared, the log files were automatically stored on the USB stick.
Otherwise, please insert a USB stick, then in the menu navigate to Preferences > Maintenance > Save logs files to USB. Send all log files to your support contact, so they can view them and provide further assistance.